1. Field of the Invention
The present invention relates to a holding device for cutting an ophthalmic lens, which is used for holding a lens workpiece when an ophthalmic lens is formed. More particularly, the present invention relates to a holding device for cutting an ophthalmic lens, which is capable of cutting easily an ophthalmic lens in which the optical center of an eyesight correction region is eccentric to the geometric center of the outer circumferential circle of the lens (i.e. a decentered ophthalmic lens).
2. Discussion of the Background
Generally, in an ophthalmic lens such as a contact lens, a lens put in an eyelid, the shape of the lens is determined so that the optical center of an eyesight correction region coincides with the geometric center of the outer circumferential circle of the lens. When the ophthalmic lens is finished, a lens supporting device is used wherein a lens workpiece is supported so that the center axis of the lens workpiece coincides with the center axis of cutting, and the lens workpiece is rotated around the center axis of the lens workpiece while the lens surface is finished by a cutting tool (cutting bit).
A study for an ophthalmic lens in recent years has revealed that in consideration of the shape of a cornea or the center position of a pupil, it is sometimes effective to deflect the optical center of the eyesight correction region from the geometric center of the outer circumferential circle of the lens.
For instance, when a contact lens is fitted to an eye, the lens is apt to move toward the ear because the radius of curvature of the front surface of the cornea is larger than the radius of curvature of a portion near the ear. Further, the center of the pupil is deflected toward the nose with respect to the center of the cornea. Accordingly, it is sometimes desirable that the optical center of the eyesight correction region should be slightly deflected toward the nose with respect to the geometric center of the outer circumferential circle of the lens.
However, in order to prepare a decentered ophthalmic lens with use of a conventional holding device for cutting, it was necessary to determine the position of the holding device by turning the entirety of the holding device so as to face the cutting tool. The position-determining operations was extremely difficult and was not practical.
Use of a non-spherical lens producing apparatus is proposed in U.S. Pat. No. 5,195,407 wherein control is made to a cutting tool so that the cutting tool is moved close to and away from a lens workpiece depending on a rotation angle of the lens workpiece which is rotated around the center axis of cutting, whereby a decentered ophthalmic lens can be finished. However, the proposed apparatus had problems that it was difficult to control for driving the cutting tool, and an increase of the rotation speed of the lens workpiece caused reduction in the accuracy of controlling the driving of the cutting tool. Thus, in the conventional apparatus, it was difficult to obtain both accuracy of processing and productivity.